The overall goal of the research is to characterize the molecular mechanisms which mediate and regulate the functions of specialized plasma membranes of the digestive system of the rat, i.e., enterocyte microvillus and basolateral membranes and hepatocyte plasma membranes. Particular emphasis is on the lipis fluidity and lipid-protein interactions in these organelles. purified membrane preparations are examined by biochemical methods; fluorescence techniques, including fluorescence polarization, phase luorometry and excimer fluoresence; and studies of enzyme and transport functions. The specific aims are to characterize the mechanisms which regulate membrane lipid fluidity and, thereby, function. One category of regulatory mechanism is "metabolic", i.e., involves enzyme-mediated changes in membrane lipid composition. The effects of calium in vitro on hepatocyte plasma membranes, of a starve-prefeed dietary regimen on hepatocyte plasma membranes, and of changes in cholesterol biosynthesis in vivo on intestinal microvillus membranes fall in this category. The effects of calcium may underly its regulatory influence on a number of membrane functions, including hormone-mediated signal-transduction and active transport of sodium and potassium. the starverefeed dietery regimen ameliorates partially the cholestasis due to ethinyl estradiol in the rat. Studies are directed to devise effective dietary modulation of membrane fluidity, both as an experimental tool and as a possible therapeutic modality in cholestatic disorders. Impermeant fluorophores are applied to the estimation of the fluidity of the individual hemileaflets of the membranes. These probes permit selective evaluation of the outer hemi- leaflet fluidity of intact cells and will be used to monitor the leaflets of normal and perturbed cells. Thermotropic transitions are studied by fluorescence techniques and differential scanning calorimetry. Fluidity and lipid composition of these plasma membranes are also examined in reltion to rat age, gender, localization in the intestine and function in digestion, absorption and metabolism. The studies should help elucidate intestinal and hepatic function and disorders such as peptic ulcer, sprue, cirrhosis of the liver and toxic or alcoholic liver disease.